Tanning with lignin sulfonic acid-chromium reaction product and fatty acid salts



3,092,436 Patented June 4, 1963 TANNING WITH LIGNIN SULFONIC ACID-CHRO-MIU Ihg REACTION PRODUCT AND FATTY ACID SAL Robert G. Banner and John H.Pierce, Painesville, and John T. Chain, Chardon, Ohio, assignors toDiamond Alkali Company, Cleveland, Ohio, a corporation of Delaware NoDrawing. Filed Apr. 26, 1961, Ser. No. 105,563

16 Claims. (Cl. 8-94.26)

This invention relates toa new and improved process for the tanning ofleather and more particularly relates to a new and improved leathertanning process which utilizes a chromium-containing tanningcomposition.

This application is a continuation-impart of our eopending applicationsSerial No. 650,825, filed April 5, 1957, now abandoned, and Serial No.26,675, filed May 4, 1960, now US. Patent No. 2,995,415.

At the present time, two recognized types of tanning procedures havebeen accepted and widely-practiced in the leather industry. These arethe so-called vegetable tanning wherein the conversion of the hide toleather is accomplished with aqueous tanning solutions extracted frommaterials of vegetable origin, such as, finely-ground oak or hemlockbark or the like, and the so-called chrome" tanning which utilizesvarious reducible salts of chromium, notably alkali metal bichromates,and especially sodium bichromate, to effect the conversion of the hidesto leather.

Each of these tanning techniques has certain advan tages which hasjustified its continued use despite other somewhat seriousdisadvantages, which at the present time, have not been completelyovercome. For example, vegetable tanning produces leather andparticularly,

sole leather, of an excellent color, having wear properties andwaterproofness which are generally quite satisfactory. However,vegetable tanning is an extremely slow process and hence despite theadvantage of providing a so-called full leather of good color, theslowness of vegetable tanning has precluded its use in many applicationswhere it otherwise would be highly desirable.

On the other hand, one of the outstanding advantages of chrome tanningis the speed with which the tanning can be efiected. Moreover, chrometanned leather exhibits good wear and abrasion resistant properties.However, the principal disadvantages of conventionally chrome tannedleather, especially when used as sole leather, are that it issusceptible to distortion and at times is slippery on wet surfaces.Additionally, chrome tanned leather frequently has a poor color andgenerally does not cut well in that ragged edges are produced oncutting.

In an effort to overcome the recognized disadvantages of vegetable andchrome tanning, prior workers in the art have proposed severalvariations in tanning procedure.

For example, it has been suggested that leather first be tanned with avegetable tanning material and then retanned with a cationic,chromium-containing tanning composition, i.e., the conventional type ofchrome tanning composition. Advantages of such a procedure are that ahigher shrinkage temperature is obtained, together with increasedabrasion resistance, especially in sole leather so prepared. However, ithas been found that leather thus produced frequently has a poor color.Moreover, since such a procedure involves additional treatment,inevitably there is an increase in the cost involved. In addition, theextremely slow vegetable tanning process, used as a first step, still isencountered.

Alternatively, prior workers in the art also have proposed first tochrome tan leather and then to use a vegetable tanning composition as aretan. However, this technique has not been found to be the completeanswer to the problem either, because a heavy vegetable tan frequentlyis required to obtain a satisfactory leather color. Inasmuch as thedesirable wear properties of the leather generally decrease with anincrease in the amount of vegetable retanning effected, this procedurehas resulted in leather, and particularly, sole leather, having poorWear characteristics.

In addition to the above proposals for overcoming the disadvantages ofchrome tanning and vegetable tanning, it has been proposed to tan theleather using a tanning composition containing a predominant amount ofnonionic chromium as the tanning material. Such a tanning composition isprepared by reacting lignin sulphonic acid with sodium bichromate in theamount of at least 25% by weight of the lignin solids in the ligninsulphonic acid. This method is described in detail in our copendingapplication Serial No. 650,825. Although by such a tanning process theprincipal disadvantage of chrome tanning and vegetable tanning isovercome and a leather having an abrasion resistance considerablygreater than that of either chrome or vegetable tanned leather isgenerally obtained, the resistance to water absorption of this leather,although superior to that of conventionally chrome tanned leather, isnot as good as that of vegetable tanned leather. Consequently, it isnecessary to subject the leather to a waterproofing treatment, such as,impregnation with a silicone resin. Moreover, it has been found that thequality of the leather so produced often varies from one tanning run tothe next. Thus, although the leatheer produced in one run may have goodcolor and resistance to abrasion, that produced in succeeding runs maynot.

Accordingly, it will be appreciated that up to the present timeconsiderable effort has been expended in an attempt to improve thepresently-available techniques for the tanning of leather, particularlyleather destined for application as sole leather.

It is, therefore, the principal ob 'ect of the present invention toprovide an improved leather, especially sole leather, characterized by amaximum of the desirable properties of vegetable tanned leather as wellas those of chrome tanned leather, while minimizing the disadvantages ofboth prior tanning techniques.

Another object of this invention is to provide an improved leathertanning process wherein this improved leather will be consistentlyproduced.

Another object of the present invention is to provide an improvedleather, especially sole leather, which is characterized by a resistanceto water absorption at least comparable to that of a vegetable tannedleather.

A further object of the present invention is to provide a new andimproved tanning process by means of which this improved leather can beproduced, which process is an improvement over the process set forth inapplications Serial Nos. 650,825 and 26,675.

These and other objects of the present invention will become apparent tothose skilled in the art from the description of the invention whichfollows.

The method of the present invention envisions a tanning process whichutilizes a cationic compound selected from the group consisting ofcationic salts of fatty amides, fatty amines, fatty amine esters andcyclic fatty amides in combination with a predominantly non-ionic,chromium-containing tanning composition, which composition is theproduct obtained by chemically reacting lignin sulphonic acid with atleast sufiicient of a hexavalent chromium compound to produce apredominantly nonionic, chromium-containing material.

More specifically, in the practice of the present method,

where the cationic compound used is a cationic salt of a fatty amide,which is the preferred cationic compound, the fatty amide may be furtherdefined as being the reaction product of a fat and an amine, whichreaction product is then further reacted with an ionizable, saltformingcompound to form the cationic substance. By a fat" is meant a solid orliquid oil consisting of the glyceryl esters of fatty acids and mixturesof fatty acids. The amines may be lower aliphatic amines, such as thosecontaining up to about 7 carbon atoms in a straight or branched chainand may be primary, secondary or tertiary, mono-, dior tri-amines, ormay be higher polyamines, such as triethylenetetramine ortetraethylenepentamine. The ionizable, salt-forming compounds areexemplified by aliphatic acids, acid salts, as Well as halides of bothaliphatic and aromatic compounds, such as halogenated ethers and thelike. It will, of course, be appreciated that in addition to reacting afat and an amine to prepare the fatty amide, the amide may be preparedby the reaction of an amine and a fatty acid or by the reaction of alower fatty acid ester, such as a methyl ester, with an amine orammonia.

The non-ionic, chromium-containing tanning composition is prepared byreacting lignin solids, in the form of lignin sulphonic acid, with ahexavalent chromium compound, generally sodium bichromate, the sodiumbichromate being in the amount of at least 25% by weight of the ligninsolids and not in excess of about 50% by weight of the lignin solids.The preparation of this predominantly non-ionic, chromium-containingtanning composition is given in detail in our copending applicationsSerial Nos. 650,825 and 26,675. This chromium tanning composition, whenused in conjunction with the cationic compound described hereinabove, inaccordance with the practice of the present method, is found to yieldconsistently leather of improved properties, particularly with regard toresistance to water absorption and wear, thus providing a new andimproved tanning process.

The expression lignin sulphonic acid as used throughout thespecification and claims is intended to refer to lignin sulphonic acidobtained from neutralized sulphite waste liquors. In practice, theseliquors, formed as a byproduct from the cooking of wood fibers in thepaper industry, are generally neutralized with calcium or magnesiumsalts and the resulting liquors used as such or spray dried to obtain acrude water-soluble lignin sulphonate, generally calcium sulphonates, oras termed herein, lignin solids." One commercially-available product ofthis type is termed Goulac. The water-soluble lignin solids can bedissolved in water and acidified prior to use in the practice of thisinvention, or, if desired, may be formed in situ during the preparationof the composition embodying the present invention.

In the practice of the present method, the hides which are to be tannedare subjected to the conventional pretanning treatments of soaking orwashing, liming, fleshing, dehairing, bating and pickling. Thereafter,the hides are subjected to the cationic composition in combination withan aqueous solution of the non-ionic, chromium-containing tanningcomposition obtained by chemically reacting lignin sulphonic acid with ahexavalent chromium compound. Although there is no one critical point inthe present process at which the hides must be contacted with thecationic compound and the non-ionic, chromium-containing tanningcomposition, it has been found to be essential that at the time thehides are contacted with the cationic compound and the non-ionic,chromium-containing tanning composition, a reaction between thesematerials can take place. Thus, for example, the cationic compound canbe added to the sodium chloride solution with which the hides are wettedand in which they are fioated prior to the addition of the non-ionic,chromiumcontaining tanning agent. Alternatively, the cationic compoundand the non-ionic, chromium-containing tanning composition can be addedsimultaneously to the hides,

or, as a further alternative, the cationic compound can be added to thehides immediately after tanning with the non-ionic, chromium-containingagent has been completed. However, it has been found that when the hidesare contacted with the cationic composition after the non-ionic,chromium-containing tanning material has been removed therefrom or atthe time of some subsequent treatment, such as, during retanning or fatliquoring, at which time no appreciable reaction between the cationiccompound and the non-ionic, chromium-containing tanning agent takesplace, the leather thus produced may be of a poorer quality,particularly with regard to color, fullness, and resistance to waterabsorption.

While it is not known for certain why a leather having superiorcharacteristics can be consistently produced only when the cationiccomposition and the non-ionic, chromium-containing tanning agent arebrought into contact with the hides at a time when a reaction betweenthe materials can take place, it is known that the cationic compositionand non-ionic, chromium-containing tanning agent do react to give aproduct, which, when dry is only very slightly soluble in water and thecommon organic solvents, and only slowly soluble in acids or alkalies.It is believed that the formation of this product in the hides which aretanned is responsible for the production of the leather having theabove-described superior characteristics. It is for this reason that itis essential in the process of the present invention that the hides arecontacted with the cationic material and the non-ionic,chromium-containing tanning agent at a time in the process wherein areaction between these materials can take place. It has further beenfound that from the standpoint of penetration of the hides andcompleteness of reaction, it is preferable to add the cationiccomposition to the sodium chloride wetting or float solution prior tothe addition of non-ionic, chromium-containing tanning agent.

It has further been found, in the practice of the present method, thatthe addition of a small amount of a buffering material such as sodiumformate or to similar organic acid salts, the non-ionic,chromium-containing tanning material, e.g., in the amount of about 2%%by weight of the fleshed hides, results in faster penetration of thetanning composition into the hides, thus speeding up the rate oftanning. Moreover, when sodium formate is added to the tanning solution,it is no longer necessary to raise the pH of the tannage by the additionof alkali, such as, sodium bicarbonate, in order to obtain a good degreeof tanning. Thus, where previously it has been necessary to raise thetannage pH to within the range of about 4.5 to 5.0, by adding sodiumformate to the tanning composition, leather having characterisitcs atleast as good as that previously produced and in most instances evenbetter, can be produced using a pH of about 3.0 to 4.0. It is apparentthat by this use of sodium formate the tanning of the hides isaccomplished in a shorter time.

The leather thus-produced may be used without further tanning treatmentsor, as desired, it may be retanned with a conventional vegetable tanningmaterial or any of the conventional chrome tanning materials.Additionally, the leather may be fat liqwored prior to usage, eitherwith or without the additional retannjng. It is, of course, appreciatedthat retannage of the leather produced in accordance with the presentmethod will be determined by the characteristics that are necessaryaccording to the end use of the leather. Inasmuch as the characteristicsimparted to leather by use of the conventional vegetable and/or chromiumretanning are well known to those skilled in the art, it is not deemedto be necessary to give a detailed explanation as to when the variousretannages are desirably used. Similarly, the various processes for fatliquoring leather and the characteristics imparted to leather which hasbeen so treated, are also deemed to be well known to those skilled inthe art, so that no further explanation of these processes is seen to benecessary.

In the practice of the preferred method of the present invention, thehides to be tanned, after the conventional preliminary treatments ofwashing, liming, fleshing, dehairing, baring and pickling, are floatedin an aqueous solution containing about to by weight of sodium chloride.The weight of the aqueous solution used is approximately equal to theweight of the hides to be tanned. In addition to the sodium chloride,the aqueous float solution also contains up to about 3% by weight of thehides, preferably about 0.3% to 1.2% of the cationic compound, whichcompound is preferably in the form of an oil emulsion so as to bedispersed more easily in the aqueous float solution. The hides are runin the aqueous float solution for a period up to about one hourwhereupon to by weight of the predetermined nonionic,chromium-containing tanning material and up to about 2 /2 by Weight ofsodium formate is added thereto. Taming of the hides is carried out at atemperature of about 100 F. for a period of time sufiicient to eifectcomplete penetration of the hides, generally about 4 to 8 hours.Thereafter, the hides are removed from the aqueous solution and held fora period of about 8 hours, whereafter they may be subjected to furthertreatment, such as, retanning, with a vegetable tan, retanning with aconventional chrome tanning liquor, fat liquoring, or they may be washedand dried and used wtihout further treatment. The leather thus producedis found to have a good color as well as exceptional resistance toabrasion and water absorption. It is to be understood that all of theweights given above are based on the weight of the fleshed hides.

In order that those skilled in the art may better understand the methodof the present invention and the manner in which it may be practiced,the following specific examples are given.

EXAMPLE 1 Preparation of Cationic Compounds 50 parts by weight of primelard oil and 25 parts by weight of dipropylene triamine are heatedtogether for a period of 3 hours at a temperature in the range of 220-240 F. Thereafter the reaction mixture is cooled to a temperature of 170F. and 25 parts by weight dichloroethyl ether is added thereto whilemaintaining the tempcrature below 300 F. After the addition of thedichloroethyl ether, the temperature of the reaction mixture ismaintained within the range of 290-3l0 F. for a period of 2 hours. Theresulting cationic compound is found to have the following structuralformula:

Using the procedure of Example 1, the following cationic compounds areformed:

Examples Components J Amounts Prime Lard Oil 50 parts by wgt.

Dipropylene Thiamine .l 25 parts by wgt. Acetic Acid 25 parts by Wgt. 3Prime Lard Oil..

50 parts by wgt. 25 parts by wgt. 25 parts by wgt.

These compounds are found to have the following structures:

group represents the a-cryl radicals of the fatty acids present in thelard oil, consisting substantially wholly of stearic, palmitic, oleicand linoleic acids, R being approximately C H EXAMPLE 5 To illustratethe oil-emulsion composition of the cationic compound, the followingcomposition is formed:

Percent by wgt.

Product of Example 1 30.0 Mineral oil (solvent refined petroleum oilhaving a Saybolt viscosity of 200 seconds at 100 F.) 39.7 Fleshing oil(animal oil pressed from fleshing grease) 24.0 Polyethylene glycolmono-oleate 6.0 Diamyl phenol 0.3

This composition is easily dispersed in water and is thus found toprovide an excellent way of bringing the cationic compound into thoroughContact with the hides to be tanned.

EXAMPLE 6 Preparation of the Non-Ionic Chromium Tanning Composition Onehundred lbs. of lignin solids is dissolved in 250 lbs. (30 gallons) ofWater and 35 lbs. of by weight sulphuric acid is added thereto and theresulting mass is agitated for 1 hour, during which time S0 fumes areevolved. Twenty-five lbs. of Na Cr O ll-i o as a 50% by weight solutionis then added slowly with continuous and vigorous agitation to avoidformation of any chromelignin gel lumps. After all the bicromatesolution is added, the mass is agitated for 2 hours after which time 5.5lbs. of sodium hydroxide is added, as a 25 solution, with continuingvigorous agitation. The resultant mixture is allowed to stand overnight,after which time another 5.5 lbs of sodium hydroxide, as a 25% solution,is added with vigorous agitation and sufficient water is then added tomake a total volume of 50 gallons. The pH of the resultant liquidtanning solution is 4.0. Chemical analysis of the liquid tan so preparedis as follows:

Percent by wgt.

Solids 28.6 Cr O Anionic chromium 23.9 Non-ionic chromium 70.7 Cationicchromium 5.4

This material can be employed either as a liquid or spray dried,advantageously with the desiccating chamber of the dryer adjusted so asto provide an inlet air temperature of about 400 F. and an outlet airtemperature of about 2l0230 F. during the drying operation.

EXAMPLE 7 lbs.) and 1.6% by weight of the stock of the composition ofExample 5 (5.0 lbs). The tanning drum is run for a period of about 1hour while the temperature of the float solution is maintained at about70 F. Thereafter, the non-ionic, chromium-tanning composition of Example6, as a 30% solution, is added to the drum in the amount of about 40% byWeight of the stock (126 lbs.). Additionally, 2% by weight of the stockof sodium formate (6.25 lbs.) is also added to the tanning drum.Thereafter, the drum is run for a period of about 4 hours at atemperature of about 95 F., at the end of which time the pH of thetanning solution is about 3.4. The stock is allowed to remain in thedrum in the tanning solution overnight. The liquids are then drainedfrom the tanning drum and the tanned hides are washed. The leatherthus-produced, which has a shrink temperature of about 195 F., is foundto have excellent color as well as excellent resistance to abrasion andwater.

EXAMPLE 8 Leather produced in accordance with the procedure of Example 7is retanned with a commercial chromium-tanning composition indicated bythe manufacturer, Diamond Alkali Company, as Tanolin KXD. The analysisof this material is as follows:

Percent by wgt.

S (hydrolyzable) 21.1

Na SO Basicity (basicity is defined as the percent of total chromiumwhich is present which is linked to hydroxyl groups) 46-48 The leatheris floated in the tanning drum in 34 gallons (283 lbs.) of water at atemperature of about 100 F. This amount of water is about 90% by weightof the leather in the drum. Four percent by weight of the leather in thedrum, of Tanolin KXD" (12.5 lbs.) dissolved in 2 gallons of water isthen added to the drum. The drum is run for about 2 hours at atemperature of about 100 F. at the end of which time the pH of thetanning solution is about 2.5. 0.5%, by weight of the leather in thedrum, sodium bicarbonate (1.57 lbs.) is then added to the drum and thedrum is run for an additional 30 minutes. The pH of the tanning solutionis about 3.3. The leather is then removed from the drum and rinsed. Thethus-produced leather is found to have excellent color and excellentresistance to abrasion and water absorption.

EXAMPLE 9 The leather tanned in accordance with the procedures set forthin Examples 7 and 8 is retanned using a vegetable tan. The leather isfloated in a tanning drum in 314 lbs. of water at about 70 F. Thisamount of water is equal to about 100% of the weight of the leather.23.5 lbs. quebracho, containing about 60% by weight tannin, is thenadded to the drum. This amount of quebracho is equal to about 4 /z%, byweight of the leather in the drum, of tannin. The drum is then run forabout 2 hours after which time the leather is rinsed until the rinsewater from the leather is clear. The thus-produced leather is found tohave excellent color as well as excellent resistance to abrasion andwater absorption.

EXAMPLE 10 Leather produced in accordance with the preceding threeexamples, Examples 7, 8 and 9, is loaded in an oil wheel using aconventional load consisting of sugar, mineral oil, sulfonated cod oiland fillers, for a period of about 45 minutes. At the end of this time,substantially complete penetration of the load into the leather has beenaccomplished.

EXAMPLE 11 A number of tanning runs are made using the procedure setforth in Example 7 with the exception that the amount of the compositionof Example used is varied in each of the runs. The leather obtained fromthese runs is then subjected to a Water absorption test to determine theeffect that variations in the amount of the composition of Example 5 hasupon the leather produced. In the water absorption test, a leathersample is weighed before and after being immersed in distilled water fora period of /2 hour and a period of 2 hours. The water absorption isthen expressed as the percent of weight increase of the sample afterimmersion. The exact test procedure used is Method 13-30 as set forth inthe December 31, 1957, edition of the Official Technical Methods of theAmerican Leather Chemists Association. Using this procedure, the resultsobtained are as follows:

Water Absorption in percent wgt. increase of the ample 5 in percent by\vgt. of the stock After )4 hour After 2 hours From these results it isseen that there is a decrease in the water absorption of the leatherwhen the amount of the composition of Example 5 used is increased from01% and 4% by weight of the stock being tanned. However, when the amountof the composition of Example 5 used is increased to about 12% by weightof the stock to be tanned, there is an increase in the water absorptionof the leather produced.

While there have been described various embodiments of the invention,the methods described are not intended to be understood as limiting thescope of the invention as it is realized that changes therewithin arepossible and it is further intended that each element recited in any ofthe following claims is to be understood as referring to all equivalentelements for accomplishing substantially the same results insubstantially the same or equivalent manner, it being intended to coverthe invention broadly in whatever form its principle may be uilized.

What is claimed is:

1. In the tanning of leather, the method which comprises subjecting thematerial to be tanned to a cationic compound selected from the groupconsisting of cationic salts of fatty amides, fatty amines, fatty amineesters and cyclic fatty amides in combination with an aqueous solutionof a predominantly non-ionic, chromium-containing reaction productobtained by chemically reacting lignin sulphonic acid with a hexavalentchromium compound.

2. In the tanning of leather, the method which comprises the step ofsubjecting the material to be tanned to a cationic compound selectedfrom the group consisting of cationic salts of fatty amides, fattyamines, fatty amine esters and cyclic fatty amides in combination withan aqueous solution of a predominantly non-ionic, chromiumcontainingreaction product obtained by chemically reacting lignin sulphonic acidwith a hexavalent chromi um compound, in an amount within the range of25% to about 50% by weight of the lignin sulphonic acid, expressed aslignin solids.

3. In a tanning of leather in accordance with claim 1, the further stepwhich comprises retanning the thus-treated material with a vegetabletan.

4. In the tanning of leather in accordance with claim 1, the furtherstep which comprises retanm'ng the thustreated material with a chrometan.

5. In the tanning of leather in accordance with claim 3, the furtherstep which comprises fat liquoring the thus-treated material.

6. In the tanning of leather in accordance with claim 4, the furtherstep which comprises fat liquoring the thus-treated material.

7. A tanned animal skin having incorporated therein a substantiallywater-insoluble product formed by the reaction of a cationic compoundselected from the group consisting of cationic salts of fatty amides,fatty amines, fatty amine esters and cyclic fatty amides and apredominantly non-ionic, chromium-containing composition formed bychemically reacting lignin sulphonic acid with a hexavalent chromiumcompound.

8. A tanned animal skin having incorporated therein a substantiallywater-insoluble product formed by the reaction of a cationic compoundselected from the group consisting of cationic salts of fatty amides,fatty amines, fatty amine esters and cyclic fatty amides and apredominantly non-ionic, chromium-containing composition obtained bychemically reacting lignin sulphonic acid with a hexavalent chromiumcompound in an amount within the range of 25% to about 50% by weight ofthe lignin sulphonic acid, expressed as lignin solids.

9. A process of tanning leather which comprises subjecting a pickledhide to a cationic compound selected from the group consisting ofcationic salts of fatty amides, fatty amines, fatty amine esters andcyclic fatty amides in combination with an aqueous solution of thereaction product obtained by chemically reacting a solution of ligninsulphonic acid with a solution of sodium bichromate in an amount of atleast 25% and not more than 50% by weight of the lignin solids contentof the lignin sulphonic acid, retaining said hides in said solution atan elevated temperature until the desired tanning is effected andthereafter washing the thus-tanned leather.

1 0. The process as claimed in claim '9 wherein the aqueous solution ofthe reaction product of lignin sulphonic acid and sodium bichromate alsocontain up to about 2.5% by weight of a buffering material.

11. The process as claimed in claim wherein the buffering material is anorganic acid salt.

12. The process as claimed in claim 11 wherein the organic acid salt issodium formate.

13. A method of tanning which comprises floating a pickel-d hide in analkali metal chloride solution, which solution also contains up to about3% by weight of the hides of a cationic compound selected from the groupconsisting of cationic salts of fatty amides, fatty amines, fatty amineesters and cyclic fatty amides, adding thereto a liquid tanningcomposition comprising the reaction product of lignin sulphonic acid anda hexavalent chromium compound in an amount Within the range of 25% toabout by weight of the lignin sulphonic acid expressed as lignin solids,said liquid tanning composition also containing up to about 2.5% byweight of sodium formate, retaining said hides in said solution at atemperature between about to F. for a period of time sufficient toobtain a high leather shrink temperature and thereafter Washing thethus-treated leather.

14. The process according to claim 13 wherein the thus-treated leatheris retanned.

15. The process as claimed in claim 13 wherein the cationic compound ispresent in an amount within the range of about 0.3% to 1.2% by weight ofthe pickled hides.

16. The method as claimed in claim 15 wherein the sodium formate ispresent in an amount of about 2.5% by weight of the aqueous tanningsolution.

References Cited in the file of this patent UNITED STATES PATENTS2,201,041 Katz May 14, 1940 2,328,901 Grimm et al Sept. 7, 19432,729,534 Ehrensperger Jan. 3, 1956 2,809,088 Meister et al. Oct. 8,1957 2,938,892 Sheehan May 31, 1960 2,989,362 Heyden et al. June 20,1961 2,995,415 Banner et a1. Aug. 8, 1961

1. IN THE TANNING OF LEATHER, THE METHOD WHICH COMPRISES SUBJECTING THEMATERIAL TO BE TANNED TO A CATIONIC COMPOUND SELECTED FROM THE GROUPCONSISTING OF CATIONIC SALTS OF FATTY AMIDES, FATTY AMINES, FATTY AMINEESTERS AND CYCLIC FATTY AMIDES IN COMBINATION WITH AN AQUEOUS SOLUTIONOF A PREDOMINANTLY NON-IONIC, CHROMINUM-CONTAINING REACTION PRODUCTOBTAINED BY CHEMICALLY REACTING LIGNIN SULPHONIC ACID WITH A HEXAVALENTCHROMIUM COMPOUND.